A steam turbine has an outer casing and a rotor which is rotatably mounted in the outer casing. During operation of the steam turbine, steam enters the outer casing, the enthalpy of this steam is dissipated on a blading of the rotor such that the rotor is driven in rotation.
The rotor has a rotor shaft which is mounted at each of its longitudinal ends with a bearing which is provided in the outer casing. Since, during operation of the steam turbine, the rotor shaft moves relative to the outer casing, at least one sealing gap is provided between the rotor shaft and the outer casing such that the rotor shaft does not rub against the outer casing. Due to the overpressure of the steam within the steam turbine, steam leaks through the sealing gap. Efforts are made to seal this sealing gap as effectively as possible such that the leakage of steam through this sealing gap is as small as possible. This means that the necessary shaft seal(s) must be created such that no steam escapes outward and no air can penetrate into the interior of the steam turbine.
FIG. 1 shows a conventional steam turbine 101 with an outer casing 102, a rotor 103 and a rotor shaft 104. At one longitudinal end of the rotor shaft 104, this shaft has, between a bearing and the process side of the steam turbine 101, a bearing-side shaft section 105 and a process-side shaft section 106. The transition from the bearing-side shaft section 105 to the process-side shaft section 106 consists of a shaft step 107, wherein the bearing-side shaft section 105 has a smaller outer diameter than the process-side shaft section 106. The process-side shaft section 106 of the rotor shaft 104 passes through the outer casing 102, defining a seal region 108. The seal section 108 is sealed with a shaft seal 109 which has a bearing-side labyrinth 110, a process-side labyrinth 111 and an intermediate labyrinth 112. The labyrinths 110 to 112 are provided on the process side shaft section 106.
Between the bearing-side labyrinth 110 and the intermediate labyrinth 112 there is provided, in the outer casing 102, a duct for the removal of vapor steam 114, and between the intermediate labyrinth 112 and the process-side labyrinth 111 there is provided a duct in the outer casing 102 for the supply and/or removal of barrier steam 113. The labyrinths 110 to 112 are for example in the form of a see-through labyrinth, a full labyrinth, a stepped labyrinth or a comb-groove labyrinth.
As an alternative to labyrinth seals, it is known to use see-through seals for steam turbines. However, both labyrinth seals and see-through seals require a cost-intensive leakage and barrier steam system due to the leakage quantities incurred.